Conventionally, in a battery charger equipped with a attachment part capable of connecting thereto a plurality of battery packs, a number of configurations have been adopted by considering the performance of a power supply circuit in the battery charger and the safety in case of a failure due to, e.g., a dust clogging. In accordance with such conventional configurations, a plurality of battery packs are loaded at the same time; one battery pack is loaded while another is being charged; or a plurality of battery packs are connected before a power of the battery charger is turned on. Further, when a plurality of battery packs are loaded, one of them is preferentially charged.
In accordance with an examples of the above configurations, if a battery pack is loaded after another battery pack has been loaded first and started being charged, the charging of the first loaded battery pack is continued without being interrupted by the later loaded one. Further, in accordance with another example, one of attachment ports is previously set as a priority attachment port, and, if a battery pack gets connected to the priority attachment port while other battery pack connected to other attachment port is being charged, the charging is diverted to the one connected to the priority attachment port.
However, in the above-mentioned battery charger equipped with the priority attachment port, there may be a case where the charging is diverted to the battery pack connected to the priority attachment port when other battery pack is almost fully charged. In this case, if the remaining capacity of the battery pack connected to the priority attachment port is small, it takes a long time to complete the charging of the other battery pack.
Further, if the battery pack connected to the priority attachment port enters a temperature waiting mode during being charged, the charging is stopped until the temperature of the battery pack returns to a normal temperature. Therefore, it takes an even larger amount of time for all the battery packs to be fully charged.
Further, in conventional battery chargers, there is also known a configuration in which, if a plurality of battery packs are loaded, the charging is started from a battery pack having a greater rated capacity. However, in this configuration, it requires a large amount of time to completely charge a battery pack compared to the case where the charging is started from a battery pack having a smaller rated capacity.
In this regard, Patent Document 1 discloses a method of charging battery packs as follows: reading data of full charge capacity and remaining capacity from a non-volatile memory in each of battery packs; assigning priorities to the battery packs in ascending order of difference between the full charge capacity and the remaining capacity; and charging the battery packs according to the priority. (Patent Document 1) Japanese Patent Application Publication No. 11-285159
However, in the method of the above patent document, there is a drawback in that the priorities are determined based on the remaining capacity data stored in the non-volatile memory in the battery pack. That is, it cannot be guaranteed that the remaining capacity data stored in the non-volatile memory indicates exactly the actual remaining capacity of the battery pack, especially if a long period of time has elapsed since the use of the battery pack. Therefore, it is difficult to assign the priorities to the battery packs exactly in descending order of closeness to the fully charged states.